The present invention relates to an eddy current distance meter and, more particularly, to an eddy current distance signal apparatus with compensating means for compensating for changes in temperature.
A conventional eddy current distance meter is a noncontact distance meter used when an object to be measured is a conductor. An eddy current distance meter has a primary coil and two secondary coils. When one of the secondary coils is located near an object such as a steel plate, an AC signal supplied to the primary coil generates a magnetic field, and an eddy current is generated on the surface of the steel plate. The eddy current causes generation of a magnetic field having magnetization opposite to that of the magnetic field generated by the AC signal. Thus, a voltage at the secondary coil located near the steel plate is lower than that of the other secondary coil, and the voltage difference corresponds to a distance between the eddy current meter and the steel plate.
When the level of a molten metal is to be measured, an eddy current distance meter, having the arrangement described above and being forcibly cooled, is lowered to a position near the surface of the molten metal. However, in practice, the secondary coil located near the molten metal is exposed to a higher temperature than the other secondary coil. As a result, the secondary coil located near the molten metal is thermally expanded and its dimensions change. An induction voltage changes accordingly. Although a change in induction voltage caused by a change in temperature is not very large, a difference between induction voltages at the secondary coils is also small. This voltage difference only slightly changes in accordance with a change in distance. Therefore, the error caused by a change in temperature largely affects the measurement.
In an ideal state, voltages induced at the two secondary coils are, in principle, identical and the difference is zero when an object to be measured is not present. In practice, however, due to the difference between coil diameters of the two secondary coils during the fabrication process, a variation in distance between the primary and secondary coils, and for other reasons, identical voltages cannot be obtained at the two secondary coils. Prior to measurement, the calibration of the meter must be checked to ensure a zero difference between the induction voltages at the secondary coils.